De-duplicate three copy-pasted report renderers behind one row pipeline and add a Markdown renderer, byte-identical output

# Task: refactor the report renderers and add Markdown `reports.py` has three copy-pasted renderers. Refactor so the row processing (name default, qty/price parsing, skipping negative qty, line totals, running total/count) lives in **one** place that all renderers use, then add a fourth renderer. Requirements: 1. `render_text`, `render_csv`, `render_html` keep their signatures and produce **byte-identical** output to today for every input (the grader compares against the original on hundreds of random inputs, including odd names and float prices). 2. No function in `reports.py` may be longer than 15 lines (def line to last line, inclusive), and the expression `round(qty * price, 2)` (or any `qty * price` multiplication) must appear exactly once in the file. 3. Add `render_markdown(title, rows, currency="EUR")` producing: ``` ## <title> | Name | Qty | Total | |---|---:|---:| | <name> | <qty> | <line_total with 2 decimals> <currency> | ... | **TOTAL** | <count> | **<total with 2 decimals> <currency>** | ``` followed by a final newline. In names, `|` is escaped as `\|`. Same row rules as the others (default name `(unnamed)`, negative qty rows skipped). Empty `rows` still prints the header and TOTAL lines. Standard library only. No network.